This invention relates to a radar device for scanning the frontal direction with electromagnetic waves such as laser light and millimetric waves and thereby measuring the distance to an object in front based on the reflected waves therefrom.
Radar devices of the kind for scanning the frontal direction of a vehicle with laser light or the like to determine the presence or absence of an object and to measure its distance and direction have been known. Such devices are likely to erroneously detect an object due to its faulty attachment to the vehicle or the slope of the detection area caused by the condition of the load on the vehicle.
If the radar device makes an error in the detection of an object, this may cause an error in the vehicle control device connected on the downstream side. Such a vehicle control device may be carrying on an adaptive cruise control (ACC), controlling the vehicle to follow another vehicle traveling in front (the front-going vehicle) at a constant distance based on its position data measured by the radar device. If the radar device makes an error in detecting objects, the vehicle control device may decelerate or stop the vehicle when the erroneously detected distance to an object becomes less than a specified safety threshold distance.
In view of the above, Japanese Patent Publication Tokkai 2000-329853 (Reference 1) proposed a device for correcting the displacement of an optical axis based on the position of the area where the level of reflected waves becomes greater than a specified intensity and a reference direction of the scanned area. Japanese Patent Publication Tokkai 11-64489 (Reference 2) disclosed a device for calculating the displacement based on the area where the level of reflected waves becomes a maximum.
These devices, however, are adapted to make corrections while the vehicle is at rest, not while the vehicle is in motion. A device for making a correction while the vehicle is in motion is disclosed in Japanese Patent Publication Tokkai 11-142520 (Reference 3). This device is adapted to obtain a plurality of (position) data items on the front-going vehicle while it is traveling straight forward, judge its average center position (correction position) and make a correction such that the center position of the optical axis will come to coincide with this correction position.
While in motion, however, the positional relationship between one's own vehicle and its front-going vehicle keeps changing, depending on the conditions of the travel. For example, the orientation of the own vehicle would change as it accelerates and decelerates and the positional relationship of the own vehicle and the front vehicle in the vertical direction would change according the change in the condition of the road surface such as its slope.
Japanese Patent Publication Tokkai 2004-125739 (Reference 4) discloses a technology of dynamically controlling the optical axis of laser light vertically in response to changes in the vertical positional relationship while in motion. According to this technology, a scan is executed vertically in a plurality of steps and a displacement in the vertical direction is corrected by calculating the quantity of correction of displacement in the vertical direction based on the relationship between the vertical direction at which the level of reflected waves is the largest and the direction of the center of the maximum range of the scan. Since a device according to this technology relaxes an instantaneous displacement of the optical axis when a gap on the road surface causes an instantaneous vertical oscillation, a filtering process is carried out by a moving average of the calculated quantities of displacement correction.
With a device according to Reference 3, a displacement of the optical axis can be corrected even while the vehicle is in motion. With a device according to Reference 4, the correction is possible even if there is a change in the positional relationship between the own vehicle and the front-going vehicle. With these devices, however, the correction of the optical axis cannot be effected quickly enough and the front-going vehicle is lost out of sight if there is a sudden change in the distance between the own vehicle and the front-going vehicle.
If the own vehicle approached the front-going vehicle, for example, the reflector plate on the front-going vehicle comes to a position with a relatively large angle of elevation with respect to the radar device on the own vehicle such that its optical axis is moved to an upward corrected direction. If the front-going vehicle has accelerated suddenly in this situation, the laser light may come to miss the front going vehicle since its optical axis is pointing upward. With a device according to Reference 4, the process of adjusting the direction of the optical axis downward will be carried out if the laser light is on the front-going vehicle but since the calculated quantity for the correction is subjected to a filtering process in order to reduce the variations in the direction of the optical axis due to instantaneous external disturbances, the correction of the optical axis becomes delayed. If the time constant of the filter is shortened, however, there arises the problem that the device may react too sensitively to the vertical vibrations of the own vehicle.